This invention relates to the stimulation of oil and gas wells. In another aspect, this invention relates to a novel method of hydraulically fracturing subsurface formations.
Most oil and gas naturally occurs within porous underground rock formations, and production wells are drilled directly into such natural formations. However, before such a well becomes economically feasible, the well bore itself must connect with existing natural permeability or flow channels or artificially made flow channels for the reservoir fluids. Well stimulation normally involves the treating of such oil and gas containing formations in a manner which will increase the ability of the formation rock to conduct fluid into the well.
Many techniques have been developed for hydraulically fracturing formations traversed by a well bore in order to increase the productivity of such formations. The technique of hydraulic fracturing includes applying fluid pressure to exposed formation rock until failure occurs. After a fracture is formed, the continued application of fluid pressure extends such fracture outward from the point of failure. In some cases, the opening of one or more flow channels or fractures, will in itself greatly increase the permeability of the formation. However, in the majority of treatments, it is also necessary to deposit a propping agent in the open fracture in order to prevent it from closing off by the natural overburden pressure and thereby substantially reducing the advantage gained by the initial opening of the fracture.
Various fracture fluids have been developed not only to transmit the hydraulic fracture pressure into the formation but also to transport propping agents such as sand and the like into the formation. Conventionally such fracture fluids are either Newtonian type natural fluids such as water or oil, or viscous low friction drop fluids such as pseudoplastic fluids or pseudoplastic thixotropic fluids. The latter types of fluids have often been utilized as the fracture fluids containing propping agent because they are capable of suspending and carrying the proppant and will yield high apparent viscosities at low shear rates in laminar flow and their use will not result in high friction losses during the pumping and fracturing operation.
Generally, the conventional fracture fluid when pumped into the formation will follow the course of least resistance and thus either widen existing fractures or open the most easily fracturable incipient fracture(s), and in general cause failure of the weakest part of the formation rock adjacent the well bore. This action can be undesirable where large accumulations of oil and gas or fractures communicating with such accumulations are positioned parallel to existing fractures in the formation which communicate with the well bore, or in situations where such accumulations are positioned within or beyond a zone which is difficult to fracture. Various techniques have been utilized in an attempt to divert the fracture fluid from the more permeable portions of the formation into the least permeable portions of the formation. Typically, the more permeable zones have been blocked with a build-up of solids, or a settable fluid which is pumped into the more permeable opening and then allowed to cure. Other techniques have been utilized to seal more permeable portions of the formation but all these techniques suffer from the drawback of only sealing the area adjacent the intersection of the more permeable portion and the well bore and continued pumping of the fracture fluid usually results in a diversion of the fracture fluid around the intersection block and back into the more permeable zone. Therefore, little, if any, fracturing will occur in other areas of the formation.